Tag Archives: spectral bands

Sentinel-2 Image of Plymouth from 2016. Data courtesy of Copernicus/ESA.

Sentinel-2B wasÂ launched at 01:49 GMT on the 7th March from Europeâ€™s Spaceport in French Guiana. Itâ€™s the second of a constellation of optical satellites which are part of the European Commissionâ€™s Copernicus Programme.

Its partner Sentinel-2A was launched on the 23rd June 2015, and has been providing some stunning imagery over the last eighteen months like the picture of Plymouth above. We’ve also used the data within our own work. Sentinel-2B carries an identical Multispectral Imager (MSI) instrument to its twin with 13 spectral bands:

4 visible and near infrared spectral bands with a spatial resolution of 10 m

6 short wave infrared spectral bands with a spatial resolution of 20 m

3 atmospheric correction bands with a spatial resolution of 60 m

With a swath width of 290 km the constellation will acquire data in a band of latitude extending from 56Â° South around Isla Hornos, Cape Horn, South America to 83Â° North above Greenland, together with observations over specific calibration sites, such as Dome-C in Antarctica. Its focus will be on continental land surfaces, all European islands, islands bigger than 100 square kilometres, land locked seas and coastal waters.

The satellites will orbit 180 degrees apart at an altitude of 786 km, which means that together they will revisit the same point on Earth every five days at the equator, and it may be faster for parts of southern Europe. In comparison, Landsat takes sixteen days to revisit the same point.

With all Copernicus data being made freely available to anyone, the short revisit time offers opportunities small and micro Earth Observation businesses to establish monitoring products and services without the need for significant investment in satellite data paving the way for innovative new solutions to the way in which certain aspects of the environment are managed. Clearly, five day revisits are not â€˜real-timeâ€™ and the spatial resolution of Sentinel data wonâ€™t be suitable for every problem.There is joint work between the US and Europe, to have complementarity with Landsat-8, which has thermal bands, and allows a further opportunity for cloud-free data acquisitions. Also, commercial operators provide higher spatial resolution data.

At Pixalytics weâ€™re supporters of open source in both software and imagery. Our first point of call with any client is to ask whether the solution can be delivered through free to access imagery, as this can make a significant cost saving and allow large archives to be accessed. Of course, for a variety of reasons, it becomesÂ necessary to purchase imagery to ensure the client gets the best solution for their needs. Of course, applications often include a combination of free to access and paid for data.

Nextâ€™s week launch offers new opportunities for downstream developers and weâ€™ll be interested to see how we can exploit this new resource to develop our products and services.

In our recent blog we described the five simple steps to select, download and view LandsatLook Natural Colour Images. However, did you know that the Natural Colour Image isnâ€™t actually a single image? Instead, itâ€™s a combination of three separate images!

This is because remote sensing works by exploiting the fact that the Earthâ€™s surfaces, and the substances on it, reflect electromagnetic energy in different ways. Using different parts of the electromagnetic spectrum makes it possible to see details, features and information that arenâ€™t obvious to the naked eye. Some remote sensing satellites carry instruments that can measure more than part of the electromagnetic spectrum, with each different measurement known as a spectral band.

Landsat 8 currently has two instruments, measuring eleven different spectral bands:

Three visible light bands that approximate red, green and blue

One near infrared band

Two shortwave infrared bands

Two thermal bands used for sensing temperature

Panchromatic band with a higher spatial resolution

The two final bands focus on coastal aerosols and cirrus clouds.

Combing the red, green and blue bands produces a single image that is very similar to what your eye would see; and this composite is the Natural Colour Image product that Landsat offers. However, you can also create your own colour composites using Image Processing Software, as Landsat offers the possibility of downloading an image for each of the individual spectral bands, known as the Level 1 GeoTIFF files.

Once imported into an image processing package, itâ€™s straightforward to create different composites by combining different variations of the spectral bands. For example, combing the red, green and blue bands creates an image like the one at top of the blog showing the eastern edge of Paris, with the Bois de Vincennes, the largest public park in Paris, on the left hand side.

This image has colours your eyes expect to see, for example, trees are green, water is blue, etc, known as a true colour or RGB composite. Combining other spectral bands produces images where the colours are different to what you would expect, these are known as false colour composites. As they use different parts of the electromagnetic spectrum, the surface of the earth reacts differently to the light and allows features hidden when showing true colour to become far more prominent.

An example of a false composite can be seen on the right, it uses the near infrared, red and green bands. Like in the RGB image, the park is easily distinguishable from the surrounding Paris; but in the false colour image, the parkâ€™s water features of the Lac Daumesnil and the Lac des Minimes have become visible as black swirls.

A second example of a false colour composite is shown on the right, which this time combines the near infrared, shortwave infrared 2 and the coastal aerosol band. In this case, the vegetation of Paris appears orange and jumps out of the image when compared to urbanisation shown in blue.

Using different combinations of spectral bands is just one remote sensing technique to create valuable information and knowledge from an image. However, every satellite measures different spectral bands and you need to be aware of what you are looking at. For example, weâ€™ve described Landsat 8 in this blog, previous Landsat missions have measured similar, but slightly different spectral bands; full details of all Landsat missions and their spectral bands can be found here.

Using the individual spectral bands, rather than relying on the set Landsat products, means you may gain new insights into the area you are looking at and you can great some fantastic images. You can literally make things appear before your eyes!

This week the European Space Agency announced the latest mission in the Project for OnBoard Automony (PROBA) mini-satellite programme. Proba-3 is planned to launch in four years; and will be a pair of satellites flying in close formation, 150m apart, with the front satellite creating an artificial eclipse of the sun allowing its companion views of the solar corona; normally only visible momentarily during solar eclipses.

Tamar estuary captured in October 2005, data courtesy of ESA.

The Proba missions are part of ESAâ€™s In-orbit Technology Demonstration Programme, which focuses on testing, and using, innovative technologies in space. Despite Proba-3â€™s nomenclature, it will be the fourth mission in the Proba programme. The first, Proba-1, was launched on the 22nd October 2001 on a planned two year Earth observation (EO) mission; however despite the planned lifecycle, thirteen years later it is still flying and sending back EO data. Itâ€™s in a sun synchronous orbit with a seven-day repeat cycle and carries eight instruments. The main one is the Compact High Resolution Imaging Spectrometer (CHRIS), developed in the UK by the Space Group of Sira Technology Ltd that was later acquired by Surrey Satellite Technology Limited. CHRIS is a hyperspectral sensor that acquires a set of up to five images of a target, with different modes allowing the collection of up to 62 spectral wavebands.

Plymouth, where Pixalytics is based, and our lead consultant, Dr Samantha Lavender, have a long history with Proba-1. Rame Head point, along the coast from Plymouth, is one of the test sites for the CHRIS instrument and she’s been doing research using the data it provides for over a decade. Over Plymouth Mode 2 is used, which focuses on mapping the water at a spatial resolution of 17m; this mode was proposed by Sam back in the early days of CHRIS-Proba. The image at the top of the page, captured in October 2005, shows the Tamar estuary in the UK that separates the counties of Devon and Cornwall; for this image CHRIS was pointed further North due to planned fieldwork activities. At the bottom of the image is the thick line of the Tamar Road Bridge and below it, the thinner Brunel railway bridge. Plymouth is to the right of the bridge, and to the left is the Cornish town of Saltash.

Proba-2 was launched in 2009, carrying two solar observation experiments, two space weather experiments and seventeen other technology demonstrations. ESA returned to EO for the third mission, Proba-V, launched on the 7 May 2013; the change in nomenclature is because the V stands for vegetation sensor. It is a redesign of the â€˜Vegetationâ€™ imaging instrument carried on the French Spot satellites; it has a 350m ground resolution with a 2250km swath, and collects data in the blue, red, near-infrared and mid-infrared wavebands. It provides worldwide coverage every two days, and through its four spectral bands it can distinguish between different types of land cover. The image on the right is from Proba-V, showing the Nile delta on 2nd May 2014.

Despite their small stature all the Proba satellites are showing their resilience by remaining operational, and they’re playing a vital role in allowing innovative new technologies to be tested in space.